Within the manufacturing industry, pallet based work methods are common. In these cases, mounting and/or processing take place at a number of work stations along a process flow. Since mounting and/or processing take place with a stationary product the possibility to supply a number of work stations with products for processing is ensured, in spite of the fact that the products are stationary at a work station, by means of arranging independent side transport flows to a main flow, which continuously may feed products to and from said work stations, at which side transport flows products may be removed from the main flow in order to reach a work station and also be reintroduced to the main flow from the work station.
A realization of said conveyor system with a main flow and side flows consists of module based conveyor systems where an optional number of work modules, comprising a first and a second transverse conveyor line and a longitudinal conveyor line, are connected to a main conveyor line. The first transverse conveyor line is linked to the main conveyor line via a transfer station where goods or pallets may be moved from the main conveyor line to the first transverse conveyor line. The second transverse conveyor line is linked to the main conveyor line via a transfer station where goods or pallets may be moved from the second transverse conveyor line to the main conveyor line. The first and second transverse conveyor lines are connected via transfer stations to the longitudinal conveyor line. By means of its design, the work module admits transport from the main conveyor line to a work station arranged along the longitudinal conveyor line. Examples of module based conveyor systems are described in U.S. Pat. Nos. 5,947,259 and 5,884,746.
Module based conveyor systems comprising work modules according to the description above, are used in the manufacturing industry for electronic products, such as, for example, mobile phones, printed circuit cards etc., which are sensitive for too fast electric discharges and induced electric currents Such products are usually called electrostatic discharge sensitive or ESDS. A deeper description of the problem with electrostatic discharges is for example provided in C62.38-1994 IEEE, “Guide to Electrostatic Discharge (ESD)” or via www.esda.org.
Induced electric currents may occur at the electronic product when the electronic product is transported past a charged object. A too fast discharge may take place If the electronic product gets into touch with a conducting material which is connected to ground. Said discharges or induced electric charges may thus damage the electronic product. In order to reduce these problems, dissipative materials are used. For definition of dissipative materials, it is referred to Swedish Standard SS-EN-61340-5. Examples of dissipative materials are provided in EP 434 011 on page 2 and forward, and in U.S. Pat. No. 6,140,405 below “Background Art”.
A dissipative material sees to that a discharge of an electrically charged object takes place slowly. This means that the magnitude of the current at the discharge is limited, thus preventing damage on electronic products when contacting the dissipative material. It is further seen to that the electronic products are discharged by means of contact with a dissipative material. When only electrically insulating materials are used, there is a risk that an induced current arises at the electronic product if it is transported through an electric field generated by an object placed along any of the conveyor lines where the electronic product is travelling. The dissipative material also sees to that other objects, except the electronic products, which are placed on the dissipative material are discharged.
According to previously known technology, dissipative materials are used at work modules solely within a limited work area situated behind the longitudinal conveyor. The electronic product that will be processed, controlled or handled in any other way, is placed on the dissipative surface, after which processing or control will take place. Since only a limited area within the work module is covered with a dissipative material, problems remain, since an operator not easily may distinguish between dissipatively protected areas and not protected areas. Further, according to previously known technology, the conveyors are not dissipatively protected, which means that possibly protected areas may not be broken through by conveyor lines. This means that there is a risk that the operator places the electronic products outside the work area with the dissipative material, on an electrically conducting material, risking the consequence of a too fast discharge of the product. Further, there is a risk that the operator places electrically charged objects, for example tools of different kinds, on an electrically insulated support in the vicinity of the work area. These objects will then not be discharged, why there is a risk that that electronic components may be damaged by the electric field that surrounds the charged object. Tools may also be electrically charged when they are placed outside the work area if the tool gets in contact with an electrically charged area. When a charged tool later is used in order to treat the electronic components, there is a large possibility that the tool damages the electronic product that shall be treated.